The disclosure concerns a compact electrohydraulic drive, which is especially but not only for use under water and for the rotational driving of an output element, such as a propeller, a wheel, or a cable winch. For this, the compact unit comprises as its components a hydraulic motor, having an output shaft, a hydraulic pump, by which the hydraulic motor can be supplied with a hydraulic fluid by a working line, and an electric motor, by which the hydraulic pump can be driven.
For many activities under water in connection with the extraction of fossil fuels such as oil and gas, the mining of mineral resources, the natural sciences, robotics with the aid of remote operated vehicles (ROV) or automated underwater vehicles (AUV), infrastructure programs, or renewable energy, special machines and equipment with underwater systems are required, which can be driven and controlled in the harsh environment.
Many underwater devices need to be outfitted with a controllable drive system or several controllable drive systems, also known as thrusters. Usually these thrusters have a propeller as their output element. In particular, underwater robots such as ROVs or AUVs require a thruster or several thrusters, for example eight thrusters, in order to make possible the needed mobility of the robot in all six degrees of freedom.
The output element need not be a propeller. Instead, a wheel can also be the output element, which stands on the sea bottom or is used to drive a chain, such as is the case with an underwater mining vehicle. Furthermore, the compact drive can also drive a cable winch, used for positioning of ships or underwater devices. Examples which may be mentioned are mooring, laying anchors, compensating for wave action, or stretching a chain under water.
Devices designed especially for operation under water must function safely and reliably. The customary solution is to employ hydraulic motors to drive output elements, which can be supplied with hydraulic fluid with a collective hydraulic pump across electrically actuated underwater valves. Such a device is known from GB 2 181 040 A. If the pump fails, the underwater robot or the underwater device can no longer move and must be hauled to the surface at once for repairs.
In order to compensate for the movements and currents of the water, an underwater drive furthermore requires a dynamic control system. At the same time, the drive should work with minimal energy consumption at all times. These two requirements of high dynamics and energy efficiency are hard to achieve with the systems mentioned above. Furthermore, the requirements on functional safety become greater, so as to be able to work safely even under complex conditions of application. For example, one may mention here for functional safety a safely reduced speed when approaching an object.
An electrohydraulic drive which is designed for use under water and in which a hydraulic pump driven by an electric motor and a hydraulic motor are interconnected in a closed hydraulic circuit is known from DE 29 24 364 A1. The hydraulic motor drives a propeller, constituting the output element. A total of three electrohydraulic drives of the mentioned kind are disposed on a piece of mining equipment and serve for the moving of the device under water or the moving of parts of the device relative to a device frame.
The problem which the disclosure proposes to solve is to modify an electrohydraulic drive which is provided for use under water and which comprises as its components a hydraulic motor, having an output shaft, a hydraulic pump, by which the hydraulic motor can be supplied with a hydraulic fluid by a working line, and an electric motor, by which the hydraulic pump can be driven, so that it is especially suitable for use under water.
This problem is solved by a compact electrohydraulic drive in which the components are present in a closed container filled with hydraulic fluid and the container has an opening for coupling of the output shaft of the hydraulic motor with the output element. Thus, according to the disclosure, the compact electrohydraulic drive forms a self-enclosed unit, which houses the entire electric motor, hydraulic pump, and hydraulic motor layout. The compact drive combines the benefits of the high power density of the hydraulics with a decentralized direct electric drive. High reliability and safety are achieved. The handling of the drive is easier. The electric motor, which is coupled to the hydrostatic transmission formed by the hydraulic pump and the hydraulic motor, can be a small, light and compact electric motor running at high rotary speed. Advantageously, the electric motor has variable rotary speed control.
In order to achieve especially high dynamics of the drive, the hydraulic pump is advantageously adjustable in its stroke volume. Preferably, the hydraulic motor is also adjustable in its stroke volume, so that the dynamics and the energy efficiency of the electrohydraulic drive are further enhanced.
Advisedly, an electrical control system for the power supply of the electric motor is arranged in the container. Advantageously, the electrical control system is outfitted with computing capacity and is programmed with algorithms for the operation of the components, or is suitable to being programmed with algorithms for the operation of the components.
It is advantageous if the hydraulic motor can also be operated as a hydraulic pump, the hydraulic pump can also be operated as a hydraulic motor, and the electric motor driven by the hydraulic pump working as a hydraulic motor can also be operated as a generator. Then electrical energy can be recovered, driven by a propeller or a wheel, and stored for later consumption in a battery. For this, algorithms for the recovery and storage of energy are integrated in the electrical control system.
The compact drive may contain an electronic control module with built-in functionality for a dynamic positioning control of a vehicle which is outfitted with the drive. The control module can be an integral module of the electronic vehicle control system.
Preferably sensors are integrated in the compact electrohydraulic drive, especially sensors such as sensors for the pressure in the hydraulic circuit, for the rotary speed, for the position, for the velocity, for the acceleration, for the temperature and for the condition, for example, for the degree of contamination of the hydraulic fluid and for the water depth, especially microelectromechanical sensors (MEMS). By monitoring the temperature, one can avoid the drive from failing when operating in an environment with extreme temperatures, such as may prevail above the surface of the water. With sensors, the stroke volume of the hydraulic units can be detected. Likewise, with sensors the rotary speed of the components can be detected for an especially good control capability. An algorithm can be implemented with which the output torque of the hydraulic motor is limited, so as to avoid damaging the output element, especially a propeller. Likewise, an algorithm can be implemented with which the pressure in the hydraulic circuit is increased in order to free the propeller from an object which has gotten caught in it.
Regulating functions may be integrated for the automatic compensation for external disturbances such as, e.g., water currents or opposing forces when activating an actuator. For example, sensors are used to measure the accelerations on the drives. The propellers are then operated so that the forces along the axis of orientation of the propeller are reduced as much as possible by generating opposing forces of equal strength. Likewise, algorithms can be integrated for condition monitoring, such as, for example, a counting of operating time or a monitoring of torque and vibration. Algorithms can be integrated for maximizing the dynamics and the efficiency.
A compact electrohydraulic drive according to the disclosure advantageously has at least one communications interface for exchanging data with or without cable.
Safety functions can be integrated in the electronic control system as closed regulation circuits.
In particular, the following safety functions are conceivable:                a) Safe torque off (STO): when the electrical control system receives an emergency off command via the communications interface, the electric motor and thus also the hydraulic pump and the hydraulic motor are switched off (without power). The propeller will stop after an uncontrolled time and an uncontrolled path.        b) Safe stop 1 and 2 (SS1 and SS2): when the electrical control system receives a particular command (such as SS1 or SS2 messages) via the communications interface, the electric motor and thus also the hydraulic pump and the hydraulic motor are actuated so that the propeller is halted after a controlled maximum time and a controlled maximum path.        c) Safe maximum speed (SMS), safe limited speed (SLS): The electrical control system regulates the rotary speed of the propeller with corresponding sensors (such as encoders) so that the rotary speed does not exceed the maximum value dictated by the communications interface. If this value is exceeded, the electric motor will be switched off. Besides the maximum rotary speed, the control system can regulate a temporary reduced velocity, for example to make possible a certain sensitive movement. This function enables, for example, a close approach to an object.        d) Safe direction (SDI): when the electrical control system receives a command via the communications interface to move safely in a particular direction, this rotary movement of the propeller is monitored by means of a sensor. If the wrong direction of rotation is indicated, the electric motor is switched off, for example in order to move out from a dangerous area.        e) Safe maximum torque (SMT): the torque of the propeller is regulated by a corresponding sensor or corresponding sensors (for example, a pressure sensor and a stroke volume sensor). If the torque exceeds the maximum predetermined value, the electric motor is switched off.        f) Safe stopping system in the orientation of the propeller axis: the external forces acting on the drive are measured with an acceleration sensor and the electric motor is actuated so that an opposing force is generated, so as to maintain the position in this way. If the acceleration nevertheless exceeds the predetermined value, another safety function is activated, for example, “safe direction of rotation” or “safe disconnected torque”. By arranging several differently oriented compact drives on an underwater robot (such as AUV or ROV), the position of the robot in several directions can be controlled and maintained by the combination of this function.        g) Safe communication (SCO): the transmission of safety-relevant data, such as commands or parameters via the communications interface, is monitored with corresponding fault recognition methods. If a fault is recognized, the electrical control system initiates a safety function, such as “safe disconnected torque”.        
The compact electrohydraulic drive can have at least one interface by which hydraulic fluid can be replenished or replaced under water.
Advantageously, electrical and mechanical interfaces of a compact electrohydraulic drive according to the disclosure can be decoupled under water. This makes it possible to replace a compact drive with the help of a diver or a robot (remotely operated vehicle or autonomous underwater vehicle).
Advisedly, the compact electrohydraulic drive has a filter or several hydraulic filters with or without status sensors, for example in order to prevent excessive contamination of the hydraulic fluid with water or particles. The status sensors can indicate whether a replacement of the hydraulic fluid is necessary.
For use at greater depths, the container of a compact electrohydraulic drive has a movable compensation piston, which bounds the interior space of the container by a first surface and is subjected to the ambient pressure at a second surface, which is just as large as the first surface and oriented opposite to it. If, for example, an additional force is exerted on the compensation piston by a spring in the direction of the interior space of the container, the pressure in the container will always be somewhat higher than on the outside, so that no water can get in. By monitoring the position of the compensation piston, one can identify a leakage of hydraulic fluid to the outside.
A set of at least two compact electrohydraulic drives according to the disclosure can be arranged on a device to be moved, wherein the movement of the device is realized by operating of the compact electrohydraulic units in coordination with each other, with or without an overarching control system.
If a vehicle is outfitted with several compact electrohydraulic drives according to the disclosure, i.e., a set of compact electrohydraulic drives according to the disclosure, it is possible by an intelligent algorithm or regulation circuit to compensate as much as possible for the failure of one compact unit by the operation of the other compact units.